Solid-state high-luminance far ultraviolet light emitting element including highly pure hexagonal boron nitride single crystal

ABSTRACT

A solid-state far ultraviolet light emitting element is formed by a hexagonal boron nitride single crystal, excited by electron beam irradiation to emit far ultraviolet light having a maximum light emission peak in a far ultraviolet region at a wavelength of 235 nm or shorter.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional application of Ser. No. 10/566,722 filed on Feb. 2,2006, now abandoned, which is based on PCT/JP2004/017434 filed on Nov.17, 2004.

FIELD OF THE INVENTION

The present invention relates to (i) a solid-state high-luminance farultraviolet light emitting element including highly pure hexagonal boronnitride single crystal.

BACKGROUND OF THE INVENTION

Development of high-luminance ultraviolet light emitting elements isrecently progressing toward practical use. Light emitting elements withthe emission wavelength of the order of 300 nm have been proposed usingvarious materials such as gallium nitride and solid solution thereof.For the changeover to the shorter wavelength of the emitting wavelengthsof these solid-state light emitting elements, there is a large demand infields such as high densification of recording media and others. Todate, as the candidates for far ultraviolet light emitting element on awavelength of the order of 200 nm, diamond, cubic boron nitride crystal(hereinafter, denoted by cBN) and aluminum nitride have been proposedand studies for application thereof are in progress.

In searching for materials for a high-luminance light emitting elementin the far ultraviolet region, important characteristics include: havingbroad band gaps and chemical stability, and preferably to be directtransition type semiconductors, and the like. Except for above describedmaterials, the solid-state light emitting materials with far ultravioletlight emission characteristics of the order of 200 nm include hexagonalboron nitride crystal (hereinafter, denoted by hBN) having about 5.8 eVband gap and being a direct transition type semiconductor. But therehave been factors to prevent its realization. hBN has been used for along time as a chemically stable insulator material, is synthesized bygas phase reaction of boron oxide and ammonia, and is now utilized inmany forms (such as powder, sintered body, and film form).

However, hBN obtained by the above described gas phase reaction hascontained impurities to make it difficult to obtain hBN having the farultraviolet light emission characteristics corresponding to its specificband gap. In order to use this material as the high-luminance lightemitting element in far ultraviolet region, it is necessary first toestablish methods to synthesize highly pure single crystals, but therehas been no report until now that the highly pure hBN single crystalwith expected light emission characteristics has been successfullyobtained by a hBN synthesizing method, aiming at its potential abilityas a solid-state far ultraviolet light emitting element with theemission wavelength of the order of 200 nm.

As for the synthesizing method, hBN has been known to be synthesized bythe thermal decomposition reaction or by the gas phase reaction betweenboron compounds such as boron oxide and ammonia, but it has beendifficult to obtain highly pure single crystals by these reactions.Especially, they have never been considered established as themanufacturing methods of single crystal materials to use forsemiconductors or the like.

On the other hand, cubic boron nitride crystal, a high-pressure phase ofhBN, has been known to be synthesized by using hBN or the like as theraw material and boronitride of alkali metal or alkali earth metal as asolvent and by recrystallizing said raw material in said solvent underhigh-temperature and high-pressure of 55,000 atmospheric pressure and1,600° C. Obtained cBN single crystal has high hardness next to thediamond, and is widely used as a super hard material, and this procedurefor synthesizing cBN has already been established industrially.

Because cBN synthesized in this way also has a broad band gap (e.g. 6.3eV), it has been studied for a long time as a solid-state shortwavelength light emitting element. However, every cBN single crystalhitherto reported is colored in amber, orange or the like, and the lightemitting behavior corresponding to cBN specific band gap has not yetbeen able to be observed in this situation. As a possible cause thereof,large effect of impurities contained in the cBN crystal may benominated. Therefore, in order to use the cBN single crystal as amaterial having specific light emission characteristics corresponding tothe band gap of said crystal, establishment of synthesis reaction toachieve higher purification of cBN single crystal has become animportant subject to study, as well as full understandings of lightemission characteristics specific to cBN.

Under such background, it has been reported that synthesis of hBN singlecrystal was tried under the condition of cBN synthesis daringly changingthe temperature and pressure conditions to those at which hBN isproduced stably (non-patent literature 1). However, from thecrystal-growth solvent used in the synthesis experiment in this report,only colored cBN crystals were obtained, and about the hBN crystal thatwas formed concurrently as a by-product, there was no description on thelight emission behavior thereof at all or no suggestion on shortwavelength light emission thereof.

In such a situation, the inventors of the present invention intensivelystudied the synthetic conditions for obtaining highly pure cBN singlecrystals. Consequently, they found factors necessary to obtain highlypure cBN single crystals, and thus succeeded in synthesizing highly purecBN single crystals having optical characteristics specific to the cBNcrystal, and reported in an academic literature (non-patent literature2). This synthetic procedure was, in short, after establishing a cleanand dry nitrogen-atmosphere, crystals were grown using highly puresolvent (such as barium boronitride) purified with utmost care. By thisprocedure, highly pure cBN single crystals were successfully obtained(non-patent literature 2).

-   Non-patent literature 1; H. Akamaru, A. Onodera, T. Endo, O.    Mishima, J. Phys. Chem. Solids, 63, 887 (2002).-   Non-patent literature 2; T. Taniguchi, S. Yamaoka, J. Cryst. Growth,    222, 549 (2001).

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

Above is the present situation of the hBN material or cBN, thehigh-pressure phase thereof, expected to have light emissioncharacteristics in far ultraviolet range. Especially, hBN, a wide bandgap semiconductor, is of direct transition type, and so is expected as ahigh-luminance and short wavelength solid-state light emitting element,but the present situation is as described above. In order to live up tothe above expectation, it is urgent to derive the originalcharacteristics of the substance, that is, to establish methods forsynthesis of highly pure single crystals not being affected bycontaminations, and it is expected to be realized.

Moreover, as light emitting apparatuses in the ultraviolet region, laserapparatuses using various kinds of gases or semiconductor light emittingapparatuses are known so far. But these apparatuses need cooling unitsand are large scale apparatuses, and are complicated and expensive withpn-junction, pin-junction and the like, so that ultraviolet lightemitting apparatuses being simple, compact, low cost, and highlyefficient are desired.

The present invention intends to meet these requirements. That is, theproblem to be solved by the present invention is to synthesize highlypure hBN single crystals which has been impossible to be produced by theconventional hBN synthetic procedure, and using these, to provide anelement capable of far ultraviolet high-luminance light emissionreflecting the characteristics specific to hBN. Moreover, making use ofsaid hexagonal boron nitride crystal having specific light emissioncharacteristics, the invention intends to provide simple, compact, lowcost, and highly efficient solid-state far ultraviolet lasers andsolid-state far ultraviolet region high-luminance light emittingapparatuses, instead of conventional large-scale apparatuses using gasesor complicated and expensive semiconductor apparatuses. That is to say,the invention intends to provide solid-state light emitting apparatusesutilizing the far ultraviolet solid-state light emitting elementsadopting highly pure hexagonal boron nitride crystals having farultraviolet light emission characteristics as active mediums.

Means for Solving the Problems

For the above purpose, the inventors of the present invention studied onthe synthesis experiments reported in the above non-patent reference 2,for obtaining highly pure cBN single crystals starting from the raw hBNmaterial using a clean and dry nitrogen atmosphere and purifiedsolvents. They have tried experiments to survey in detail and to controlthe critical conditions to synthesize highly pure cBN single crystals,and have found that highly pure hBN single crystals can be obtained byappropriately adjusting conditions of the temperature and the pressure.

They then surveyed in detail the optical characteristics of said highlypure hBN single crystal obtained from the above findings, and found andclarified the following optical characteristics. That is, obtainedcrystals were colorless, transparent and highly pure crystals with highelectric resistance. When the crystal was excited by electron beamirradiation with cathode luminescence, markedly high-luminance lightemission at a wavelength of 215 nm was observed at room temperature.Also, at 83 K, light emission was observed at the wavelength of 210 nmto 235 nm. According to a light absorption experiment, an absorptionspectrum showing light absorption at 208 nm and 213 nm was obtained.When this was compared with ultraviolet light emission from a highlypure diamond single crystal measured under the same condition, it wasfound that light emission intensity at the wavelength of 215 nm of thehBN single crystal at room temperature showed a value about 1000 timesor more stronger than the diamond.

That is, as the result of intensive studies for obtaining highly purehBN single crystals on the basis of prior arts described in the abovereferences (non-patent literatures 1 and 2) as the preceding techniques,the present invention succeeded in the synthesis of highly pure hBNsingle crystals having a single light emission peak in the farultraviolet region near a wavelength of 215 nm responding to electronbeam irradiation only, by setting the hBN single crystal growingconditions to the reported synthetic conditions for obtaining highlypure cBN single crystals in the above non-patent literature 2.

Also, utilizing the above highly pure hexagonal boron nitride crystal asa light emitting element or a light emitting layer, and configuring andincorporating thereinto an exciting means by an electron beamirradiation, the inventors of the present invention have succeeded ineasily designing and providing a simple, small-sized, highly efficient,solid-state far ultraviolet light emitting apparatus, unlike aconventional large-scale solid-state laser apparatus using gas necessaryto be equipped with a water cooler, or conventional light emittingapparatus using a costly semiconductor solid-state light emitting deviceproduced by repeating multiple layers of complicated pn-junctions andpin-junctions.

The present invention has been prosecuted based on a series of the abovedescribed findings and successes, and its embodiments are described inthe following (1) to (15). Among them, a group of the inventionsconcerning highly pure hexagonal boron nitride single crystals,synthetic methods thereof, and light emitting elements consisting ofsaid single crystals, given in (1) to (7), are denoted by the firstgroup inventions. Also, the inventions concerning solid-state laserscapable of emitting far ultraviolet laser lights given in (8) and (9),wherein light emitting elements comprising said single crystals arecombined with electron beam emitting means, are denoted by the secondgroup inventions. Further, the inventions concerning the solid-statelight emitting apparatus generating far ultraviolet light given in (10)to (15), wherein the light emitting layer consisting of said singlecrystal and the exciting means are integrally incorporated into a vacuumchamber, are denoted by the third group inventions.

(The First Group Inventions)

(1) Highly pure hexagonal boron nitride single crystals with farultraviolet light emission characteristics emitting far ultravioletlight having the maximum light emission peak in the far ultravioletregion at a wavelength of 235 nm or shorter.

(2) The highly pure hexagonal boron nitride single crystals with the farultraviolet light emission characteristics described in (1), whereinsaid far ultraviolet light is far ultraviolet light having the maximumlight emission peak at a wavelength of 210 nm to 220 nm, remarkably at215 nm.(3) A method for producing highly pure hexagonal boron nitride singlecrystals with far ultraviolet light emission characteristics,characterized in that the highly pure hexagonal boron nitride singlecrystals with far ultraviolet light emission characteristics emittingfar ultraviolet light having the maximum light emission peak in the farultraviolet region at a wavelength of 235 nm or shorter are producedthrough the procedures of mixing the boron nitride crystals with ahighly pure solvent, melting it by heating under high-temperature andhigh-pressure, and recrystallizing it.(4) The method for producing highly pure hexagonal boron nitride singlecrystals with the far ultraviolet light emission characteristicsdescribed in (3), wherein said far ultraviolet light has the maximumlight emission peak at a wavelength of 210 nm to 220 nm, remarkably at215 nm.(5) The method for producing highly pure hexagonal boron nitride singlecrystals with the far ultraviolet light emission characteristicsdescribed in (3) or (4), wherein said solvent is selected from nitrideor boronitride of alkali metal or alkali earth metal.(6) A solid-state far ultraviolet light emitting element consisting of ahighly pure hexagonal boron nitride single crystal, excited by electronbeam irradiation to emit far ultraviolet light having the maximum lightemission peak in the far ultraviolet region at a wavelength of 235 nm orshorter.(7) The solid-state far ultraviolet light emitting element described in(6), wherein said far ultraviolet light is a single-peakedhigh-luminance light with the peak at a wavelength of 210 nm to 220 nm,remarkably at 215 nm.(The Second Group Inventions)(8) A solid-state far ultraviolet laser characterized by the generationof laser light with a far ultraviolet region wavelength, using a highlypure hexagonal boron nitride crystal with far ultraviolet light emissioncharacteristics as a direct-type semiconductor solid-state lightemitting element and combining therewith an electron beam irradiationapparatus as an exciting source.(9) The solid-state far ultraviolet laser described in (8), wherein saidlight in the far ultraviolet region generated thereby is thesingle-peaked high-luminance laser light with a peak at a wavelength of210 nm to 220 nm, remarkably at 215 nm.(The Third Group Inventions)(10) A solid-state far ultraviolet light emitting apparatus,characterized in that a light emitting layer consisting of a highly purehexagonal boron nitride single crystal capable of emitting farultraviolet light with a single emission peak in far ultraviolet regionat a wavelength of 235 nm or shorter and an exciting means for excitingsaid light emitting layer are combined and encapsulated together into avacuum container, and the light emitting layer is excited to emit farultraviolet light by operation of the exciting means.(11) The solid-state far ultraviolet light emitting apparatus describedin (10), wherein said far ultraviolet light has a single peak at awavelength of 210 nm to 220 nm, remarkably at 215 nm.(12) The solid-state far ultraviolet light emitting apparatus describedin (10), wherein said exciting means for exciting the light emittinglayer is an electron beam emitting means.(13) The solid-state far ultraviolet light emitting apparatus describedin (12), characterized in that said exciting means by the electron beamemitting means consists of an anode electrode attached to the backsurface of the light emitting layer consisting of a hexagonal boronnitride crystal, an electron beam emitting substrate attached to thelight emitting layer through an insulating spacer, a cathode electrodeattached to the back surface of the electron beam emitting substrate,and a means to apply voltage between both electrodes; and an electronbeam is emitted from said electron beam emitting substrate toward thelight emitting layer by application of voltage between both electrodes.(14) The solid-state far ultraviolet light emitting apparatus describedin (13), wherein said electron beam emitting substrate attached throughsaid insulating spacer is a diamond substrate.(15) The solid-state far ultraviolet light emitting apparatus describedin (14), characterized by the structure of said diamond substratewherein a large number of pyramid-shaped protrusions for emitting theelectron beam are arranged in a lattice-like manner on the surfacefacing the light emitting layer.

Effects of the Invention

In the present invention, the first group inventions make it possible tocreate hexagonal boron nitride single crystals having specific lightemission characteristics showing a strong and high-luminance lightemission at a wavelength of 235 nm or shorter, particularly at 210 nm to220 nm, remarkably at 215 nm in wavelength, not obtained by the priorart. Hereby, designing of solid-state high-luminance ultraviolet lightemitting elements have become possible and various requirements for suchas developments of more and more highly densified recording mediums andstronger sterilization by higher power output are able to be satisfied.

Also, in the present invention, the second group inventions make itpossible to provide a compact solid-state light emitting element and asmall solid-state laser having an oscillation wavelength of around 200nm which has been difficult to be provided hitherto, by using a simplemeans to excite the element consisting of a highly pure hexagonal boronnitride single crystal with an electron beam.

Moreover, in the present invention, the third group inventions make itpossible to provide a solid-state high-luminance light emittingapparatus compact, low cost, highly efficient, long lived, and having asingle peak at the wavelength of 210 nm to 220 nm, especially at 215 nmat room temperature, by using a highly pure boron nitride crystal as theemitting layer and by incorporating integrally this emitting layer andan exciting means, especially an electron beam exciting means utilizinga substrate having an electron beam emitting part consisting of adiamond, into a vacuum chamber.

As described above, the present invention has succeeded in providing thecompact solid-state light emitting element and the compact solid-statelight emitting apparatus having the oscillation wavelength of 210 to 220nm, especially at 215 nm, which has been difficult hitherto to berealized, and is expected to contribute largely to the development ofvarious industrial fields. The compact, high power output, low cost andlong lived solid-state far ultraviolet light emitting element and thesolid-state laser, or the solid-state light emitting apparatus aredesired in many fields, and the range of their utilization has broaddivergence such as the field of semiconductors (for making thephotolithography highly minute), the field of the information (nextgeneration high capacity optical discs), the field of the medical careand living body (opthalmological treatment, DNA cleavage and the like),and environmental field (sterilization and the like), and the benefitobtained therefrom may be immeasurable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic condition diagram showing the region for synthesisof recrystallized hBN.

FIG. 2 shows an example of light emission spectra excited by electronbeams at room temperature.

FIG. 3 shows an absorption spectrum and a light emission spectrumexcited by the electron beam at low temperature.

FIG. 4 shows a laser-oscillation spectrum excited by the electron beam.

FIG. 5 shows excitation current dependence of the laser-oscillationspectrum excited by the electron beam.

FIG. 6 shows excitation current dependence of both the light emissionintensity and the longitudinal mode width (the fringe width) excited bythe electron beam.

FIG. 7 illustrates an embodiment wherein a plane different from thelight emission output plane is excited.

FIG. 8 shows a light emission spectrum excited by the electron beam atlow temperature (83 K).

FIG. 9 illustrates a schematic diagram of a solid-state laser whereinthe laser light is generated and taken out from a parallel plate sampleexcited by an electron beam utilizing the accelerated electron beam ofan electron microscope.

FIG. 10-1 illustrates a preliminary step of a silicon substrate forproducing a diamond electron emitting device, vapor-deposited with aSiO₂ layer.

FIG. 10-2 illustrates a process wherein a photoresist pattern is formed.

FIG. 10-3 illustrates processes of SiO₂ etching and SiO₂ mask patternformation.

FIG. 10-4 illustrates a step of forming concave pyramid-shaped pits onthe Si substrate and the sectional view of the Si substrate aftercompletion of the step.

FIG. 10-5 illustrates a process to produce a diamond device by the CVDmethod using the etched Si substrate as the template.

FIG. 10-6 illustrates a sectional view of the diamond device havingprotruded structures formed after removal of the Si substrate.

FIG. 10-7 illustrates an element which is made by mounting the obtaineddiamond device on a platinum electrode substrate through a Ti/Auelectrode.

FIG. 11 illustrates the structure of a solid-state far ultraviolet lightemitting apparatus of the present invention.

FIG. 12 shows light emission characteristics of the ultraviolet lightemitting element.

EXPLANATION OF SYMBOLS

-   -   1: a solid-state laser    -   2: an electron gun using a LaB₆ filament    -   3: an accelerated electron beam flow    -   4, 6: electron beam focusing lenses    -   5: a diaphragm    -   7: an electron beam objective lens    -   8: an ellipsoidal mirror    -   9: a parallel plate of the hexagonal boron nitride crystal    -   10: ultraviolet laser light    -   11: a spectrograph    -   12: a Si substrate    -   13: a SiO₂ layer    -   14: photoresist patterning    -   15: SiO₂ layer etching    -   16: etching of Si    -   17: a diamond layer and pyramid-shaped diamond    -   18: a Ti/Au electrode    -   19: a platinum electrode    -   20: an extraction electrode of Au    -   21: a glass plate    -   22: an electron beam    -   23: far ultraviolet light    -   24: an anode electrode of Ti/Au    -   25: a substrate of a hexagonal boron nitride crystal

THE BEST EMBODIMENTS FOR CARRYING OUT THE INVENTION

Hereinafter, the best embodiments for carrying out the present inventionare explained in a sequential order from the first group inventions tothe third group inventions.

The first group invention of the present invention relates to highlypure hBN single crystals capable of emitting ultraviolet light in farultraviolet region, synthesis processes thereof, and the light emittingelement consisting of said single crystal.

The highly pure hBN single crystal capable of emitting ultraviolet lightin the far ultraviolet region is produced by processes to treat the rawmaterial of hBN at high-temperature and under high-pressure in thepresence of highly pure solvent of alkali metal or alkali earth metalboronitride, followed by recrystallization.

By recrystallization, the hBN single crystal free from impurity havinghigh-luminance ultraviolet light emission at the wavelength of 235 nm orshorter, especially at 210 nm to 220 nm, remarkably at 215 nm, can beobtained. The temperature and pressure conditions therefor needhigh-temperature and high-pressure. As a tentative guidepost, 20,000atmospheric pressure and 1,500° or higher are preferable.

These conditions are the temperature and pressure under which the rawmaterial, boron nitride, is recrystallized into hBN under thecoexistence of the solvent. Boronitride of alkali metal or alkali earthmetal used as the solvent must exist stably without oxidization ordecomposition during the process. Especially, it is effective to forwardthe reaction under high-pressure. This suppresses decomposition of thesolvent and enables crystal growth for a long time necessary for thesynthesis of large and highly pure crystals, and thus is preferable.

However, attention should be paid to the fact that under toohigh-pressure, the raw material hBN may make phase transition to ahigh-pressure phase, cBN. That is, in order to obtain intended highlypure hBN single crystals, the temperature and pressure conditions in theregion free from cBN single crystal generation are needed. FIG. 1 showsthe temperature and pressure conditions to recrystallize hBN. Accordingto this figure, recrystallization of hBN is possible even under thethermodynamically stable conditions of cBN, but transfer to cBN proceedsmore easily with the increase in pressure, and so higher reactiontemperature, the condition for stable hBN, is necessary in order toforward hBN recrystallization.

That is, as the upper limit pressure for hBN recrystallization, around 6GPa may be appropriate. Under higher pressure than this, the synthesisconditions must be set at the thermodynamically stable conditions of hBNand the temperature on this occasion is near 3,000° C. This condition isnot appropriate for obtaining crystals large enough in size. Therefore,considering the economical efficiency in industrial manufacturing, theupper limit for the synthesis condition of said single crystal may beabout 60,000 atmospheric pressure. As for the lower limit, even under 1atmospheric pressure or lower, synthesis of high-luminance lightemitting highly pure hBN crystals through recrystallization may bepossible, if decomposition and oxidization of the solvent can berepressed. In the present invention, the high-luminance light emittinghighly pure hBN crystals were synthesized in hBN recrystallizationregion shown by netting in FIG. 1.

On the other hand, boronitride of alkali metal or alkali earth metal andthe like easily react with water and oxygen. hBN recrystallized from thereaction system containing these oxides and the like as impurities wasaffected by the impurities such as oxygen and the like, and the hBNsingle crystal capable of showing light emission phenomenon in the shortwavelength region at or below 300 nm, could not be obtained. In contrastto this, the present invention can provide highly pure hBN singlecrystals showing light emission in shorter wavelength region such as ata wavelength of 235 nm or shorter, especially showing high-luminanceultraviolet light emission at a wavelength of 210 nm to 220 nm,remarkably at 215 nm, by using commercially available so-called lowpressure phase boron nitride as the raw material and by dissolutionthereof into the highly pure solvent followed by recrystallization,which has not been possible to obtain by the conventional techniques orprior arts.

Next, the first group inventions are explained specifically based onexamples and figures. However, these examples, etc. are disclosed for ahelp of easy understanding of the invention, and the present inventionis never limited by these examples and the like.

Example 1

Hexagonal boron nitride crystal sintered body (about 0.5 μm grain size)on which deoxidation processing by heat treatment in vacuum at 1,500° C.and in nitrogen gas stream at 2,000° C. had been applied, was loadedinto a molybdenum capsule in a high-pressure cell together with a bariumboronitride solvent. The preparation of the solvent and loading thesample into the capsule were all performed under dry nitrogenatmosphere. The high-pressure reaction cell was treated at the pressureand temperature conditions of 25,000 atmospheric pressure and 1,700° C.for 20 hours by a belt type high-pressure apparatus. The increasing rateof temperature was around 50° C./min. After cooling with the rate ofabout 500° C./min, the cell was decompressed and the sample wasrecovered together with the molybdenum capsule in the high-pressurecell.

The molybdenum capsule was removed by mechanical or chemical treatment(mixed solution of hydrochloric acid and nitric acid), and the samplewas recovered. Colorless and transparent hexagonal prism form crystals(around 1 to 3 mm) were obtained, and on the crystals, identification ofthe phase by optical microscopic observation, SEM observation and X-raydiffraction, and assessment of optical characteristics (transmittance,cathode luminescence) were prosecuted. It was confirmed that the crystalwas a single phase of hBN by X-ray diffraction patterns of the crystalgrains.

By the cathode luminescence observation, single-peaked high-luminanceultraviolet light emission was observed near a wavelength of 215 nm atroom temperature as shown in FIG. 2, and, an ultraviolet light emissionbands (as shown by up arrows ↑ in the figure) were observed at 210 nm to235 nm at the temperature of 83 K, as shown in FIG. 3.

In a light absorption observation, high transmittance was shown at awavelength of 2,500 nm to near 200 nm, and light absorption structures(as shown by down arrows ↓ in the figure) were observed at thewavelengths of 208 nm and 213 nm at the temperature 8 K as shown in FIG.3.

Example 2

A hexagonal boron nitride crystal sintered body (about 0.5 μm grainsize), on which deoxidation processing had been applied by heattreatments in vacua at 1,500° C. and in nitrogen gas stream at 2,000°C., was loaded into the molybdenum capsule together with the solvent ofmixed barium boronitride and lithium boronitride 1:1 by weight ratio.High-pressure treatment was applied in the same manner as in Example 1and the sample was recovered.

The recovered sample had a same morphology as in Example 1, andascertained to be hBN crystal. By cathode luminescence measurement, abroad light emission was observed near 300 nm, together with ahigh-luminance light emission at a wavelength of 215 nm.

Example 3

A hexagonal boron nitride crystal sintered body (about 0.5 μm grainsize) on which deoxidation processing by heat treatment in vacuum at1,500° C. and in nitrogen gas stream at 2,000° C. had been applied, wasloaded into a molybdenum capsule together with the solvent of mixedbarium boronitride and lithium boronitride 1:1 by weight ratio. Thepreparation of this solvent and loading of the sample into capsule wereall performed under dry nitrogen atmosphere. The molybdenum reactioncell was processed in nitrogen gas stream at the pressure andtemperature conditions of 1 atmospheric pressure and 1,500° C. for twohours. The rate of temperature increase was about 10° C./min. Themolybdenum capsule was recovered after cooling with the rate of about20° C./min.

Then, the molybdenum capsule was removed by mechanical or chemicaltreatment (mixed solution of hydrochloric acid and nitric acid), and thesample inside was recovered. The solvent portion partly showed an aspectof decomposition, but in part, recrystallization was seen at theinterface with the hBN raw material. Solvent component was removed byacid treatment. After washing, on the obtained hBN crystal,identification of the phase by optical microscopic observation, SEMobservation and X-ray diffraction was prosecuted, and assessment thereofthrough the optical characteristics tests (transmittance, cathodeluminescence) was done.

As a result, a broad light emission near 300 nm was observed by cathodeluminescence measurements together with a high-luminance light emissionat a wavelength of 215 nm.

In other cases than the above examples 1 to 3, measurements on manysamples produced under a little different synthetic conditions made itclear that the maximum luminescence peaks were concentrated inparticular at a wavelength of 210 nm to 220 nm, remarkably at 215 nm.Although these maximum luminescence peak widths are narrow, theydistribute with a considerable widths. Causes thereof are not altogetherclear, but ununiformity of the crystallinity due to defects or minorcomponents such as impurities may concern.

Comparative Example 1

Deoxidation processing by thermal treatment in vacuum at 1,500° C. andin nitrogen gas stream at 2,000° C. had been applied to the commerciallyavailable hBN sintered body and hBN powder, and then light emittingbehaviors thereof were measured by the cathode luminescence. As aresult, no single-peaked strong light emission near 215 nm was observed.

Comparative Example 2

In the case that the solvent used in the processes described in Example1 contained oxygen impurities due to oxidation in part, recrystallizedhBN single crystals could be synthesized by re-using this solvent in hBNsynthesis experiments, mixing the raw material with the solvent andtreating them with high-temperature and high-pressure. However, by thecathode luminescence measurement, a broad and strong light emission wasobserved near a wavelength of 300 nm, rather than 215 nm. It isconsidered that by the effects of impurities such as oxygen, thehigh-luminance short-wavelength light emission characteristics wereinhibited.

The above comparative example 2 instructs that recrystallization usinghighly pure solvents is important to produce the highly pure hBN singlecrystals and to make them express good high-luminance light emissioncharacteristics. These examples and comparative examples show that, inthe synthetic conditions, atmosphere and high grade purification of thesolvents used are important in producing highly pure high-luminancelight emitting hBN single crystals in the present invention.

Based on the above findings, using low pressure phase boron nitride asthe raw material and highly pure solvents such as barium boronitride,recrystallization of boron nitride was conducted. Then hexagonal boronnitride single crystals showing a behavior of single-peakedhigh-luminance light emission at a wavelength of 215 nm was obtained.

Next, the second group inventions of the present invention are explainedbased on examples and figures. However, these specific explanations aredisclosed as a help for easy understanding of the invention, and theinvention is never limited by these examples. The used materials ornumerical conditions such as impurity concentrations or film widthsdescribed in the following explanations are some of the examples only,and the invention is never limited by these examples.

Example 4

First, both faces of a parallel plate were formed by delamination alongthe cleavage plane utilizing cleavability of the c plane of the highlypure hexagonal boron nitride crystal obtained in Example 1, and aFabry-Perot etalon consisting of the parallel plate of several tens ofμm in thickness was formed.

FIG. 9 shows a far ultraviolet solid-state laser element constructedusing this parallel plate and an accelerated electron beam of anelectron microscope. In the figure, the element utilizes an electronmicroscope constructed of machine components: from the electron gun 2using an LaB₆ filament to the electron beam objective lens 7. Electronbeam flow 3 emitted from the LaB₆ filament of the electron gun wasaccelerated and incident on the c plane of said parallel plate samplewith the energy of 20 KeV and 860 mA/cm². Emitted light from the samplewas then collected by an ellipsoidal mirror 8 to be analyzed by aspectrograph 11.

As a result, it was clarified that far ultraviolet laser light aroundwavelength region at 215 nm was emitted from the sample excited by theelectron beam. FIG. 4 is the laser-oscillation spectrum at that time,from the c plane of the parallel plate sample about 10 μm in thickness.As shown in FIG. 4, there appeared sharp spectrum structures like finecomb-teeth in the light emission centering on near 215 nm. Thesespectrum structures having shapes of comb-teeth indicate thatlongitudinal modes of the Fabry-Perot etalon formed by front and backsides of the parallel plate are optically amplified by the inducedemission of the hexagonal boron nitride crystal excited by electronbeams, and it became clear that laser-oscillation operation took place.

Example 5

As in Example 4, making use of cleavability of the boron nitride singlecrystal obtained in Example 1, a parallel plate sample about 6 μm inthickness was prepared, oscillated and measured in the same way as inExample 4. FIG. 5 and FIG. 6 show results of the measurements. Accordingto these figures, due to the incompleteness of the cleavability, a laserthreshold of the electron beam density was elevated and the thresholdvalues of the laser-oscillation operation and light emission operationwere observed.

As shown in the lower figure of FIG. 6, when electron beam density(excitation current) is gradually increased, light emission outputsuddenly starts to increase more rapidly at a certain electron beamdensity. This electron beam density (excitation current value) can bedefined as the threshold value. In FIG. 5, from the spectrum with thelargest light emitting intensity to the one with the fifth intensitycorrespond to the measured points above the threshold at which lightemission output suddenly starts to increase rapidly in the lower figureof FIG. 6.

In a resonance mode of Fabry-Perot etalon, that is, a wavelengthposition of the longitudinal mode shown by in FIG. 5, these spectra showthe width-narrowing of fringe-like spectra in the excitation currentvalue range greater than or equal to the threshold value as is shown inFIG. 6 above, and shows that at each wavelength position of thelongitudinal mode the laser-oscillation operates above the thresholdvalue. In this way, with the laser-oscillation threshold value as aborderline, the element is shown to be usable as a laser element at orabove the threshold value, and as a solid-state ultraviolet lightemitting element other than a laser element below the threshold value.

Laser oscillation operation in the above mentioned example refers to thelaser-oscillation operation of the sample, the boron nitride, producedunder the specific synthetic condition obtained in Example 1, but thistype of laser-oscillation operation is not limited to the one obtainedin Example 1. Other than Example 1, similar results were observed on theboron nitride grown under the synthetic conditions of Example 2 or 3.

In the above described Examples 4 and 5, the parallel plate Fabry-Perotetalon was used. However, there is a method, wherein, instead of theparallel plate, the hBN crystal is processed into the shape ofrectangular waveguide as is shown in FIG. 7. This structure allows lightto reflect at both end faces of the waveguide to resonate. The sideface, not containing the face to take out the laser light or the emittedlight, is excited. Adopting this method, due to the fact that the faceexcited by electron is different from the faces providing laserresonator mirrors, damages such as pollution and element face brake-downof the laser end face and the excitation end face can be repressed, andalso amplification region can be set over the whole waveguide. Also, byoptimizing the shape of the light waveguide, single mode oscillations inboth transverse mode and longitudinal mode are possible.

Moreover, although a LaB₆ filament was used as the source of acceleratedelectron beam in above described Examples 4 and 5, it is possible todrastically decrease the element size by utilizing, for example, smallcathodes such as carbon nano-tube emitter or a diamond emitter.

In above mentioned Examples 4 and 5, laser-oscillation and lightemission phenomenon of the light emission band with the peak at awavelength of 215 nm were described. The light emission bands in thewavelength of 210 nm to 235 nm obtained by cooling the above mentionedsample also show laser-oscillation operation, which can be understood bythe remarkable increase in light emission intensity at each energypositions of the longitudinal mode as is shown by the spectrum in FIG.8. Thus, these bands are possible to be utilized as lasers.

In Example 4, as the acceleration energy condition of the electron beam,acceleration voltage of 20 keV and electron density of 860 mA/cm² wereadopted, but the laser-oscillation is not restricted to this condition,but should be determined by the optical loss at both end faces of thelaser resonator and the optical loss in the waveguide. With the sampleshowing the spectrum in FIG. 4, similar oscillation operation isconfirmed, for example, at the electron density of 0.2 mA/cm².

In Examples 4 and 5, the cleavage planes without modification wereutilized as reflection planes of the Fabry-Perot etalon. But it ispossible to obtain positively a high reflectivity by adopting anembodiment to deposit suitable metals (Al, MgF₂) and the like on thecleavage planes to increase the Q value of the resonator and decreasethe threshold value. This procedure may be expected as an effectivemeans.

Furthermore, in Examples 4 and 5 described above, an example wasdisclosed wherein the single crystal obtained in the embodiment of thefirst group invention was used to design a solid-state laser. Thissuggests that the boron nitride single crystal itself can be made into astructure appropriate to resonate the light. However, it is evident thatthe present invention has a function as far ultraviolet generationsolid-state light emitting elements, not restricted to the laserelement. Therefore, the present invention involves an embodiment assolid-state light emitting elements other than the laser elements. Inthis case, it is hardly necessary to say that the boron nitride crystaldoes not need to be constructed into a special structure like theresonator structure of a laser element. The single crystal has only tobe cut into a suitable size and shape, whereto an electron beam emittingapparatus is combined, and is used.

Next, the third group inventions of the present invention are explainedbased on examples and figures. However, also these examples disclosedhere are disclosed for a help of easy understanding of the invention,and the invention is never limited by them.

The third embodiments of the present invention provide specificutilization methods for the invention of the highly pure hexagonal boronnitride single crystals with far ultraviolet light emissioncharacteristics obtained in the first embodiments of the presentinvention, and propose specifically a solid-state light emittingapparatus of electron beam excitation type emitting far ultravioletlight having a single light emission peak at 215 nm.

FIGS. 10-1 to 10-7 are process drawings illustrating each of producingsteps of the electron emitting device based on a diamond substrate thatcauses the light emitting element or the light emitting layer consistingof said single crystal of the present invention to emit light. FIG. 11illustrates structure of a solid-state far ultraviolet light emittingapparatus of the present invention produced by this process and FIG. 12shows far ultraviolet light emission characteristics of this apparatus.

Example 6

Production processes of the light emitting layer consisting of thehighly pure hexagonal boron nitride crystal are disclosed here.

The highly pure hexagonal boron nitride single crystals were producedaccording to the same processes as in Example 1.

The resultant crystals were analyzed and assessed by various analyticalmeans such as identification of the phase with optical microscopicobservation, SEM observation and X-ray diffraction, and opticalcharacteristics tests (transmittance, cathode luminescence). As aresult, the crystal was ascertained to be of the single hBN phase. Bythe cathode luminescence observation, single-peaked high-luminanceultraviolet light emission was observed near a wavelength of 215 nm atroom temperature as shown in FIG. 2, and, an ultraviolet light emissionspectrum (as shown by ↑ in the figure) was observed at 210 nm to 235 nmat the temperature of 83 K, as shown in FIG. 3.

In a light absorption measurement, high transmittance was shown from thewavelength around 2,500 nm to 200 nm, and light absorption structures(shown by ↓ in the figure) were observed at the wavelengths of 208 nmand 213 nm at the temperature 8 K as shown in FIG. 3.

As the obtained single crystal had strong cleavability along the cplane, slice-shaped thin films of about several millimeters square inarea were cut out, making use of this cleavability. Thickness from theextent of several tens of μm to several μm may be enough and preferable.Ti/Au (about 15 nm in thickness) was applied on the back face thereof toform an anode, and was used as the light emitting layer in thesolid-state far ultraviolet light emitting apparatus shown in the nextExamples 7 and 8.

Example 7

Production processes of an electron emitting device made of diamond forexciting the light emitting layer obtained in Example 6 is disclosed.These processes consist of steps illustrated in from FIG. 10-1 to FIG.10-7.

As shown in FIG. 10-1, a silicon (100) substrate 12 is provided, andSiO₂ layer 13 of about 200 nm in thickness is formed on the substrate.Next, after photoresist was applied uniformly, square pits with one side70 μm in length were formed at intervals of 7 μm (FIG. 10-2) using aphotoresist pattern 14, and then naked SiO₂ part was etched 15 byhydrogen fluoride aqueous solution to form a mask pattern on the SiO₂layer 13 (FIG. 10-3). Next, concave pyramid-shaped pits consisting offour (111) planes are formed on the Si (100) substrate 12 by 15%(CH₃)₄NOH solution heated to 90° C. (FIG. 10-4).

After this photoresist and SiO₂ on the substrate are removed usinghydrogen fluoride aqueous solution or the like, a boron-added diamondplane is formed by using hot filament CVD method or the like and mixingdiborane gas (B₂H₆) to make the boron atom/carbon atom concentrationratio of about 100 ppm (FIG. 10-5). Here, as the diamond plane mustsupport itself, a thickness of about tens of μm is needed. Next, the Sisubstrate 12 forming the mold is dissolved away by a liquid mixture ofHF:HNO₃=1:1 to form a diamond substrate 17 with pyramid-shapedstructures (FIG. 10-6). Designating the face with diamond minuteprotrusion structures as the front face, a Ti/Au contact 18 for anelectrode is formed on the back face, and then the diamond plate isplaced on an electrically conductive substrate such as platinumsubstrate 19 (FIG. 10-7).

Example 8

Construction processes of a far ultraviolet light emitting apparatus(FIG. 11).

A glass plate 21 (about 100 μm in thickness) for insulation was providedon the electron emitting element produced by the procedure like inExample 7, a circular hole with a diameter of about 500 μm was formed,and gold (Au) 20 was vapor-deposited on the surface around the hole edgewith thickness of about 50 μm as shown in the figure. On thisgold-deposited plane 20, the thin film of the hexagonal boron nitridecrystal produced in Example 6 was placed so that Ti/Au-deposited facethereof contacts with the gold-deposited plane, and thus an electronemission device having the face 17 with the pyramid-shaped minutediamond protrusions as a cathode and the Ti/Au face on the hexagonalboron nitride film as an anode 24 is formed. In this case, thegold-deposited plane on the glass plate works as an extraction electrodefor the anode. The ultraviolet-emission window of this ultraviolet lightemitting element is encapsulated in a glass tube having an window ofquartz or the like, electrodes are pulled out, and the glass tube ismade to be vacuum (for example, high vacuum at 1×10⁻⁵ Torr or lower).

Example 9

Operation procedure of the far ultraviolet light emitting apparatusconstructed as described above is shown.

By grounding the electrode on the platinum substrate of the farultraviolet light emitting apparatus and applying a voltage of about 1kV or higher on the anode extraction electrode 24, electrons are emittedfrom the emitting source of the diamond pyramid-shaped minuteprotrusions 17 and excite the hexagonal boron nitride crystal 25. Theexcited hexagonal boron nitride crystal 25 exhibited light emission withthe peak at 215 nm at room temperature. The emitted ultraviolet light istaken out from the back surface of the hexagonal boron nitride crystal,and is obtained through the ultraviolet-emission window. FIG. 12 shows alight-emission spectrum (with a peak at about 215 nm and alsolight-emission bands at 300 nm) of this light emitting apparatus.

Example 10

Procedures of laser-oscillation operation of the far ultraviolet lightemitting apparatus are shown.

Experimental data already known in the art (non patent literature 3)have shown that the hexagonal boron nitride crystal plate can performlaser-oscillation by an acceleration voltage of about 20 kV when theexcitation current density is set to about 0.2 mA/cm². At theacceleration voltage of 1 kV in this case, number of pairs of anelectron and a positive hole equivalent to the above condition can beachieved at about 4 mA/cm². With the current of about 10 μA, the farultraviolet light emitting apparatus is considered to perform laseroperation.

Also, by depositing an appropriate metal (Al, MgF₂) or the like on theupper face of the cleavage plane, effects to obtain high reflectivity toincrease the Q-value of the resonator, and to decrease the thresholdvalue, are expected. Further, by using a uniform Al film instead of theTi/Au film on the lower surface of the hexagonal boron nitride crystal,similar increase in the Q-value and decrease in the threshold value areexpected.

As shown in the above examples, the present invention has succeeded inobtaining a compact and highly efficient ultraviolet light emittingelement or an apparatus completely different from conventional farultraviolet light emitting apparatuses. These examples show just onlysome embodiments thereof, and the present invention is not limited tothe above examples. For example, the far ultraviolet light emittingapparatus in the above described examples uses the boron nitrideproduced under the specific synthetic condition obtained in Example 1,and the far ultraviolet light-emission by electron beam excitation ofthis boron nitride is referred to here. However, the light emission likethis is not limited to the one obtained in Example 1. Besides Example 1,similar results were observed on the boron nitride grown under thesynthetic conditions of Example 2 and 3.

In the above examples, the diamond emitter is used as an electron beamsource, but, for example, a carbon nano-tube emitter or the like mayalso be utilized.

Moreover, as for the pyramid-shaped minute protrusions, by furtherincreasing in number, arranging in a lattice-like manner, andcontrolling individual protrusion independently, a patterned electronbeam emission and far ultraviolet light emission can be obtained andutilized for display apparatus and the like, for example.

-   Non-patent literature 3: Nature Materials, vol. 3, 404-409 (2004)

INDUSTRIAL APPLICABILITY

The present invention provides a hexagonal boron nitride single crystalshowing a strong high-luminance light emitting behavior at thewavelength of 235 nm or shorter, especially at 210 to 215 nm, havingbeen never obtained by the prior arts. Due to this, a solid-statehigh-luminance ultraviolet light emitting element has become possible tobe easily designed. In addition, it is of great significance that theinvention has provided a basic material capable of responding to recentincreasing needs for developing higher density recording media, and thepresent invention is expected to contribute largely to the developmentof industry. Also, needs for sterilization treatment by ultravioletlight have recently been taken up seriously as one of the importantenvironmental measures. The present invention provides effectivematerials therefor, and is expected to contribute to the industrialdevelopments in this aspect and to serve greatly for the improvements ofthe living environment in the future.

1. A solid-state far ultraviolet light emitting element consisting of ahexagonal boron nitride single crystal which is substantially free ofoxygen impurities, excited by electron beam irradiation to emit farultraviolet light having a maximum light emission peak in a farultraviolet region at a wavelength of 235 nm or shorter, wherein saidhexagonal boron nitride single crystal is obtained by high-temperatureand high-pressure treatment from 20,000 to 60,000 atmosphere pressure atleast 1500° C., and said high-temperature and high-pressure treatmentuses alkali earth metal boron nitride which is substantially free ofoxygen impurities or alkali metal and alkali earth metal boron nitrideswhich are substantially free of oxygen impurities as a solvent.
 2. Thesolid-state far ultraviolet light emitting element in claim 1, whereinsaid far ultraviolet light has the maximum light emission peak at awavelength of 210 nm to 220 nm.
 3. The solid-state far ultraviolet lightemitting element in claim 1, wherein said far ultraviolet light has themaximum light emission peak at a wavelength of 215 nm.
 4. Thesolid-state far ultraviolet light emitting element in claim 1, whereinsaid high-temperature and high-pressure treatment comprises arecrystallization process.